As is well known to those skilled in the art, it may be desirable or necessary to separate various components from the streams in which they are found in commercial operations. In the field of chemical processes, for example, it may be desirable to effect a separation between hydrogen and hydrocarbon gases in order to obtain one or the other or both in enriched or purified state. Prior art techniques to effect this separation include distillation--but distillation is characterized by high initial capital costs and substantial operating costs, which costs generally increase as operating conditions deviate from ambient conditions.
There is, in particular, considerable commercial interest in separating various aliphatically unsaturated hydrocarbons from mixtures containing them. These unsaturated hydrocarbons are reactive materials that serve in various roles, generally as intermediates in chemical syntheses. A number of the unsaturated hydrocarbons are employed as monomers in the formation of polymers and, in this regard, olefins such as ethylene, propylene and butadiene are well known. These olefins, as well as other unsaturated materials, for instance acetylene, are also used to form relatively low molecular weight products.
The aliphatically unsaturated hydrocarbons are most often available on a commercial basis in admixture with other chemical compounds, frequently other hydrocarbons. These hydrocarbon-containing streams are usually by-products of chemical syntheses or separation processes. When the hydrocarbon streams are liquid under normal conditions or can readily be made so, ordinary distillation techniques can be used to separate the hydrocarbon components providing they have sufficiently different boiling points for the process to be economically feasible. Especially when the hydrocarbon mixtures contain materials having close boiling points, which is often the case with hydrocarbons of the same number of carbon atoms or having a difference of only one carbon atom, distillation may not be an attractive separation procedure. In such cases, more costly processes are often used and involve operations such as solvent extraction or extractive distillation which entail considerable expense, if indeed they are technically feasible in a given situation.
Cryogenic distillation is the primary means used commercially to separate feeds which are gaseous under ambient conditions, such as olefins from hydrocarbons of the same number of carbon atoms. This type of process, however, is very costly, in terms of both capital and operating expenses, particularly for components with similar boiling ranges, such as ethylene/ethane, propylene/propane, butylenes/butanes, nitrogen/oxygen and the like.
When the mixture containing the aliphatically unsaturated hydrocarbon is in an essentially gaseous state at normal or ambient conditions of temperature and pressure, separation of the desired component from the mixture may be even more troublesome. In these situations cryogenic processes may be used, but they are expensive. The components of these normally gaseous mixtures may not even have particularly close boiling points, but nevertheless the mixture must be cooled in order to separate one or more of its components. In spite of the considerable cost of cryogenic operations, the procedure has been employed commercially for the separation of ethylene from other gaseous materials such as ethane and methane.
Evaporation of a liquid and separation of its components has been investigated somewhat thoroughly. The generally accepted theory of evaporation is that the molecules of the substance are in continuous motion and are bound to each other by various molecular forces. The energy of the molecules follows a distribution, and statistical theory holds that the distribution is generally bell-shaped. Evaporation occurs when molecules having higher than average energy escape from an exposed surface of the substance when their velocity is too great to be restrained by the molecular forces. The molecules which escape from the exposed surface also have a distribution of energies and they exert a partial pressure corresponding to the physical and chemical nature of the evaporating substance in conjunction with surrounding conditions such as pressure and temperature. The escaped molecules continue their motion, but being freed from the mother-liquid they are able to travel longer distances. The ability of these molecules to move in the gaseous state is described in terms of molecular mean free path, which is the average distance between collisions. By traveling away from the liquid, the evaporated molecules build a mass transfer boundary layer where the concentration of the evaporated substance decreases rapidly with the distance from the mother-liquid.
Formation of a liquid phase from the vapor at any pressure below saturation and/or evaporation in a porous solid have been the subjects of much theoretical and/or experimental research. Such work provides a number of simple, though necessarily idealized pore models: the cylinder, the parallel-sided slit, the wedge-shape, and the cavity between spheres in contact. For discussions of the progress of capillary condensation and evaporation in idealized pores see, for example, J. H. de Bore, "The Structure and Properties of Porous Materials" (eds. D. H. Everett and F. S. Stone), p. 68-141, Butterworths, London (1958), D. H. Everett, "The Solid-Gas Interface" (ed E. A. Flood), Vol. 2, p. 1055-1113, Dekker, N.Y. (1967), and more recently, S. J. Gregg "Adsorption, Surface Area and Porosity" 2Ed p. 116-129, Academic Press, New York (1982).
There currently also exists a number of methods for the selective separation of gaseous feed stream components, including the removal of light olefins from gas streams containing other non-olefinic components. While there has been interest in using membranes as a means of separation, the lack of high flux/high selectivity membranes economically suitable for an industrial setting has hindered the application of membranes for this purpose. Ideal characteristics for membranes for separation purposes include the ability of the membrane to maintain its integrity under high pressure and severe environmental conditions; to achieve high performance levels; to maintain high performance levels for an economical period of time in order to produce consistent and reliable results; to be easily assembled from commercially available components or manufactured with relative ease; and to be technically feasible. The present state of membrane technology offers a number of different membrane systems which attempt to achieve the various characteristics stated above. See, for example, U.S. Pat. No. 4,784,880 (November, 1988) Coplan et al., U.S. Pat. No. 4,971,696 (November, 1990) Abe et al., U.S. Pat. No. 5,104,425 (April, 1992) Rao et al., U.S. Pat. No. 5,238,569 (August, 1993) Soria et al., U.S. Pat. No. 5,415,891 (May, 1995) Liu et al., U.S. Pat. No. 5,487,774 (January, 1996) Peterson et al., U.S. Pat. No. 5,702,503 (December, 1997) Tse Tang, or U.S. Pat. No. 5,716,527 (February, 1998) Deckman et al.
Although the study of separation of gases mixtures by permeation through microporous membranes has been carried out by numerous researchers, most have not studied conditions under which capillary condensation occurs. This research has focused primarily upon membrane separation by selective permeation where one or more compounds passes through the membrane due to differences in the size or shape of the molecules. Another general class of membrane process achieves separation based upon the selective reaction of certain compounds with a reactant fluid on the other side of the membrane. A few publications have reported the permeation of capillary condensate in inorganic, microporous media. See, for example, U.S. Pat. No. 5,057,641 (October, 1991) Valus et al., U.S. Pat. No. 5,318,553 (June, 1994) Najjar et al., or U.S. Pat. No. 5,358,553 (October, 1994) Najjar et al.
In general, applying a trans-membrane mixed-gas pressure across a porous membrane may or may not cause separation of the co-permeating components. If the gas molecules collide preferentially with each other instead of the pore wall (that is, the pore diameter exceeds the bulk mean free path), a theory of viscous flow applies and no separation occurs. On the other hand, if the mean free path between collisions in a normal bulk-gas phase of equal pressure exceeds the pore size of the membrane, separation can occur. The latter process, termed "Knudsen diffusion," is promoted by operation at low pressures or by using membranes with small pores at elevated pressures. The more rapidly moving low-molecular-weight gas executes more frequent diffusional steps because it hits the wall more frequently. The ratio of wall collisions in this limit scales with the square root of penetrant molecular weight; so, the Knudsen selectivity equals the square root of the molecular-weight ratio of the largest to smallest gas. This principle was used for isotope enrichment using a non-condensable gas on the Manhattan Project, but it is uneconomical for commercial separation applications.
For more-condensable components a process, believed to involve surface adsorption, can occur on internal pore surfaces. In such condensable systems, an additional contribution to transport, termed surface diffusion, occurs in combination and even in competition with Knudsen flow. This phenomenon can be used for gas/vapor separations because it leads to blockage of the Knudsen diffusion passages that are otherwise available to noncondensable gases. Under selected conditions, high selectivities in favor of the more-condensable component have been reported.
M. B. Rao and S. Sircar, Journal of Membrane Science, 85, p. 253 (1993), reported an example of nanoporous selective surface adsorption membrane. The membrane studied had pore sizes in the range of 5-6 Angstroms, and were produced by carbonization of polyvinylidene chloride. Applying a transmembrane mixed-gas pressure across this nanoporous polymeric membrane is said to enrich a medium-purity, hydrogen-containing stream (20-60% H.sub.2) before it is further purified in a pressure swing adsorption (PSA) unit. The nanoporous polymeric membrane may also remove some highly condensable hydrocarbons that would complicate operation of the PSA unit. Hydrogen was selectively rejected in the membrane while higher-molecular-weight components, such as C3-C5 hydrocarbons, passed to the low-pressure side of the membrane. In principle, this approach might eliminate the need to recompress the majority gas, H.sub.2. By contrast, using conventional size-selective membranes, the H.sub.2 typically passes into the permeate and requires expensive recompression.
Examples of selective surface adsorption membranes based on compressed carbon pellets, Vycor glass, alumina, silica, and large pore zeolites have also been reported.
Although the study of separation of gaseous mixtures by permeation through microporous inorganic membranes has been carried out by numerous researchers, most have not studied conditions under which capillary condensation occurs. A few publications have reported the permeation of capillary condensate in inorganic, microporous media.
Ash, Barrer and co-workers report having observed the capillary condensation effect experimentally, R. Ash, R. M. Barrer and C. G. Pope, "Flow of Adsorbable Gases and Vapors in a Microporous Medium. I. Single Sorbates" and "Flow of Adsorbable Gases and Vapors in a Microporous Medium. II. Binary Mixtures" Proc. Roy. Soc A, 271, January, 1993, pp 1 to 33, and R. Ash, R. M. Barrer and R. T. Lowson, "Transport of Single Gas and Binary Gas Mixtures in a Microporous Carbon Membrane" J. Chem. Soc., Faraday Trans. I, 69 (1973) p 2166. They studied various gas mixtures in microporous carbon membranes and they described a "blocking" effect whereby the condensed phase in the membrane eliminated or "blocked" the transport of non-condensed species.
Hannong Rhim and Sun-Tak Hwang developed a model for flow of capillary condensate and measured individual permeation rates of C.sub.2 H.sub.6, n-C.sub.4 H.sub.10, and CO.sub.2 through porous Vycor glass, "Transport of Capillary Condensate", J. Colloid and Interface Sci., 52 (1) July, 1975, pp 174 to 181. Permeabilities of these gases go through maxima with increasing pressure (from 0.6 to 0.8 P sat.), then fall off rapidly as the condensate flow fills increasing amounts of the pore volume.
Later, Lee and Hwang, measured permeation of Freon 113 and H.sub.2 O through Vycor glass membranes and compared their experimental results to an improved model ("The Transport of Condensable Vapors Through a Microporous Vycor Glass Membrane", J. Colloid Interface Sci., 110(2) p 544 (1986)). Their model depended on the pressures on each side of the membrane and on independently measured adsorption isotherms. Using the Kelvin equation with corrections for adsorbed layer thickness and vapor pressure reduction in their model, they obtained good agreement between calculated and measured permeabilities.
Masashi Asaeda and Luong Dinh Du reported separation of gaseous mixtures of alcohol and water using a 10 .mu.m thick alumina membrane modified with non-calcined silicates with 3 nanometer (nm) pores, Journal Chemical Engineering, Japan, 19 (1) (1986) p 72 to 77 and 84 to 85,). The azeotropic points encountered in distillation were bypassed. At the minimum condensation pressure the concentration of alcohol in the condensate is much higher than the azeotropic concentration. Temperatures at which these membranes can be used, however, are limited to 90.degree. to 100.degree. C., due to the materials used.
R. J. R. Uhlhorn, K. Keizer and A. J. Burggraf reported separation of C.sub.3 H.sub.6 /N.sub.2 (60/40) mixtures by preferentially permeating C.sub.3 H.sub.6 through a supported .gamma.-Al.sub.2 O.sub.3 film, Journal of Membrane Science, 66, (1992) p. 259-269. Their two layer support was described as consisting of a 2 mm thick layer (pore diameter 5 .mu.m, porosity 40%), on top of which a 30 .mu.m thick intermediate layer (pore diameter 0.2 .mu.m, porosity 45%) was deposited. A 5 .mu.m thick .gamma.-Al.sub.2 O.sub.3 film top layer was synthesized on this support. Slit shape pores formed in the .gamma.-Al.sub.2 O.sub.3 film and were reported to have lengths much larger than widths theoretically comparable to the space between two parallel, infinite planes. Uhlhorn et al. state that, in this configuration, resistance to gas and vapor phase transport is determined by the .gamma.-Al.sub.2 O.sub.3 top layer only and not by the support. Helium was used downstream (and below the membrane) as a sweep gas to reduce the effect of concentration polarization. Occurrence of a maximum in the permeability was said to coincide with blocking of the pores by adsorbate. Using MgO-modified membranes to decrease the pore size, they obtained separation factors as high as 80. However, the permeability of propylene decreased by a factor of 20. They also observed hysteresis in the separation factors as a function of C.sub.3 H.sub.6 partial pressure, depending on whether the test pressure was approached from above or below.
In a recent study, David P. Sperry, John L. Falconer and Richard D. Noble separated CH.sub.3 OH/H.sub.2 mixtures in an alumina membrane with approximately 2.5-nm diameter pores, "Methanol-Hydrogen Separation by Capillary Condensation in Inorganic Membranes, J. Membrane Sci. 60 (1991) pp 185 to 193. The H.sub.2 permeability decreased by three orders of magnitude when capillary condensation of CH.sub.3 OH occurred. The CH.sub.3 OH permeation rate actually increased slightly after capillary condensation occurred. Measurements were taken up to 473.degree. K.
It is an object of this invention to provide a novel membrane system particularly characterized by its ability to separate condensable compounds from a gaseous mixture of two or more chemical compounds of differing volatilities without requiring a high differential pressure across the membrane.
Another object of the invention to provide a novel membrane system particularly characterized by its ability to separate condensable compounds from a gaseous mixture of two or more chemical compounds of differing volatilities at reduced pressures and temperatures and increased selectivity by selective condensation within the capillary structure of the membrane.
Another object of the invention is to provide a process of forming a membrane capable of separating condensable compounds from a gaseous mixture of two or more chemical compounds of differing volatilities, preferably by capillary condensation without need for cryogenic conditions of low temperature and high pressure, and/or without requiring a high differential pressure across the membrane.
Other objects and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the drawing and the appended claims.